1. Field of the Invention
The present invention relates to a method, computer method, and system for measuring structures formed on semiconductor, or other types of substrates, using X-ray images as input.
2. Description of Related Art
Production of semiconductor devices, displays, photovoltaics, etc., proceeds in a sequence of steps, each step having parameters optimized for maximum device yield. Metrology steps are used during (in-situ) and between various processing steps, to ascertain that device processing is proceeding normally and that target device parameters have been obtained, such as device physical dimensions, electrical and other properties, etc. If not obtained, metrology allows for early identification of failing processes allowing early redirection of mis-processed devices to a rework process flow, or discarding thereof.
Modern semiconductor packages pack a large number of devices into a single packaging, and one way to increase the packaging density is to use three-dimensional (3D) stacking of semiconductor die. To establish electrical connections between the individual die in such a package, through-silicon vias (TSV) are often used. A TSV is an opening created in the semiconductor die, generally from one side of the die to the other side. The TSV is typically filled with metal conductor, such as copper, to establish an electrical connection between the devices formed on the die, and another set of devices formed atop another die beneath the first die. Stacking many layers of die allows many billions of individual devices to be packaged into a package that requires a relatively small area on the circuit board atop which it is mounted.
To ensure that target electrical performance of TSVs is obtained during processing, metrology steps are used to monitor the TSV-formation processes. Because the process of formation of TSVs involves etching of very high aspect-ratio vias, monitoring the dimensions of TSVs formed on the die is of particular importance. Existing methods use electron microscopy to accomplish this task, some of which methods are destructive in nature (i.e. they require sample die to be cleaved to inspect the TSV dimensions), and some of which are nondestructive. But, the biggest issue is a process throughput reduction associated with redirecting samples for inspection, to a standalone electron microscope.
Therefore, the need exists for TSV metrology and inspection with high throughput, preferably integrated with the etching tool, and which is nondestructive. For TSVs, which are of relatively large size compared to contemporary semiconductor gate devices themselves, X-ray metrology is a candidate metrology that may provide the above benefits. One X-ray based metrology method is X-ray tomography, widely used in biological and medical sciences, which allows a full reconstruction of a three-dimensional structure formed on a semiconductor substrate. However, X-ray tomography requires that very many line-of-sight X-ray images be taken such that a full representation of the structure can be constructed, which reduces throughput, thus rendering the method unsuitable for in-situ process monitoring.
Today, advances in X-ray sources and planar pixel X-ray detectors now allow sufficient resolution for micron-scale TSVs to be inspected by simple imaging. What is needed is a method to quickly extract the dimensions of TSVs from simple X-ray images, concurrent with the etch process, such that the acquired metrology data can be used to alter etch (and other) process parameters, with the goal of maintaining target device properties during device processing.